From 55e722d6904c2385cd6a7dfd4bae41a13c2451fd Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 24 Mar 2026 04:46:00 -0400
Subject: [PATCH] WIP

---
 .../src/internal/client/reactivity/deriveds.js       |  7 ++++---
 .../svelte/src/internal/client/reactivity/effects.js |  2 +-
 .../svelte/src/internal/client/reactivity/sources.js |  3 ++-
 .../svelte/src/internal/client/reactivity/types.d.ts |  6 ++++--
 packages/svelte/src/internal/client/runtime.js       | 12 ++++++------
 5 files changed, 17 insertions(+), 13 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index 6ad7139be2..d1a5e2fbfa 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -88,9 +88,10 @@ export function derived(fn) {
 		f: flags,
 		fn,
 		reactions: null,
-		rv: -1,
-		v: /** @type {V} */ (UNINITIALIZED),
+		cv: -1,
+		rv: 0,
 		wv: 0,
+		v: /** @type {V} */ (UNINITIALIZED),
 		parent: parent_derived ?? active_effect,
 		ac: null
 	};
@@ -385,7 +386,7 @@ export function execute_derived(derived) {
  * @returns {void}
  */
 export function update_derived(derived) {
-	derived.rv = write_version;
+	derived.cv = write_version;
 
 	var old_value = derived.v;
 	var value = execute_derived(derived);
diff --git a/packages/svelte/src/internal/client/reactivity/effects.js b/packages/svelte/src/internal/client/reactivity/effects.js
index bf8ae97ca1..0f90d3a012 100644
--- a/packages/svelte/src/internal/client/reactivity/effects.js
+++ b/packages/svelte/src/internal/client/reactivity/effects.js
@@ -113,7 +113,7 @@ function create_effect(type, fn) {
 		prev: null,
 		teardown: null,
 		wv: 0,
-		rv: -1,
+		cv: -1,
 		ac: null
 	};
 
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 8f73c99268..5e7e4594b0 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -79,6 +79,7 @@ export function source(v, stack) {
 		v,
 		reactions: null,
 		equals,
+		cv: 0,
 		rv: 0,
 		wv: 0
 	};
@@ -238,7 +239,7 @@ export function internal_set(source, value, updated_during_traversal = null) {
 			}
 		}
 
-		source.wv = source.rv = increment_write_version();
+		source.wv = source.cv = increment_write_version();
 
 		// For debugging, in case you want to know which reactions are being scheduled:
 		// log_reactions(source);
diff --git a/packages/svelte/src/internal/client/reactivity/types.d.ts b/packages/svelte/src/internal/client/reactivity/types.d.ts
index 546a403434..ab458cbadd 100644
--- a/packages/svelte/src/internal/client/reactivity/types.d.ts
+++ b/packages/svelte/src/internal/client/reactivity/types.d.ts
@@ -11,8 +11,8 @@ import type { Boundary } from '../dom/blocks/boundary';
 export interface Signal {
 	/** Flags bitmask */
 	f: number;
-	/** Read version */
-	rv: number;
+	/** Check version */
+	cv: number;
 	/** Write version */
 	wv: number;
 }
@@ -38,6 +38,8 @@ export interface Value<V = unknown> extends Signal {
 	set_during_effect?: boolean;
 	/** A function that retrieves the underlying source, used for each block item signals */
 	trace?: null | (() => void);
+	/** Read version */
+	rv: number;
 }
 
 export interface Reaction extends Signal {
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 56eec8b4f3..805206b0c9 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -178,7 +178,7 @@ export function is_dirty(reaction) {
 				}
 			}
 
-			if (dependency.wv > reaction.rv) {
+			if (dependency.wv > reaction.cv) {
 				return true;
 			}
 		}
@@ -193,7 +193,7 @@ export function is_dirty(reaction) {
 		}
 	}
 
-	reaction.rv = write_version;
+	reaction.cv = write_version;
 
 	return false;
 }
@@ -324,13 +324,13 @@ export function update_reaction(reaction) {
 			// so that they are not added again
 			if (previous_reaction.deps !== null) {
 				for (let i = 0; i < previous_skipped_deps; i += 1) {
-					// previous_reaction.deps[i].rv = read_version;
+					previous_reaction.deps[i].rv = read_version;
 				}
 			}
 
 			if (previous_deps !== null) {
 				for (const dep of previous_deps) {
-					// dep.rv = read_version;
+					dep.rv = read_version;
 				}
 			}
 
@@ -462,7 +462,7 @@ export function update_effect(effect) {
 			destroy_effect_children(effect);
 		}
 
-		effect.rv = write_version;
+		effect.cv = write_version;
 
 		execute_effect_teardown(effect);
 		var teardown = update_reaction(effect);
@@ -547,7 +547,7 @@ export function get(signal) {
 			if ((active_reaction.f & REACTION_IS_UPDATING) !== 0) {
 				// we're in the effect init/update cycle
 				if (signal.rv < read_version) {
-					// signal.rv = read_version;
+					signal.rv = read_version;
 
 					// If the signal is accessing the same dependencies in the same
 					// order as it did last time, increment `skipped_deps`